1. Field of the Invention
This invention relates to a method of forming an image by modulating a flow of ions or charged particles such as toner particles with the aid of a screen having a number of fine passage openings or a screen having a character- or otherwise shaped opening.
2. Description of the Prior Art
The screen used with the present invention may be, for example, a photosensitive screen which, as described in our U.S. Application Ser. No. 480,280, comprises an electrically conductive member having a number of fine openings formed by knitting a thin metal wire or by etching or electroforming, and at least a photoconductive member overlaid on the conductive member. Such a screen may form thereon a primary latent image by a combination of charging from a corona discharger or like means and application of light such as the light from an image original. By the utilization of the electric field in the opening portion of the screen, the passage of the ion flow or charged particles are controlled or modulated in accordance with the electrostatic pattern of the primary latent image. In this manner, a latent image is formed on a recording medium which may be a chargeable sheet such as insulating paper or an image reception member such as an insulating drum, and such latent image is called a secondary latent image. The secondary latent image may directly be developed for utilization or may first be developed and then transferred to another recording medium for utilization. These techniques of electrophotography are disclosed in U.S. Pat. No. 3,582,206, U.S. Pat. No. 3,680,954 and our U.S. Application Ser. No. 480,280, and the screens described in these patents and application are of course usable with the present invention.
In an image formation apparatus using the abovedescribed method of image formation, it is necessary that both or one of the screen and the recording medium be formed into a cylindrical shape or, at least at the modulating position, formed into an arcuate shape, in order to attain high speed image formation and compactness of the apparatus. However, it has been found that the modulation of the ion flow or charged particles effected with the screen and the recording medium having different radii or different curvatures at the modulating position results in the formation of an unclear modulated image. If the screen and the recording medium are equal in curvature, no blur will be created in the formed image. However, if both the screen and the recording medium such as insulating drum or the like are cylindrically shaped, then the recording member makes it necessary that various members such as a developing device, transfer means and cleaning means be disposed around it. Thus, reduction in the diameter of the cylindrically shaped recording medium as described above is necessarily limited. On the other hand, if the retention copying is carried out wherein image formation is effected a number of times from a single primary latent image, and if the screen is an endless one, the circumferential length of the screen should preferably be equal to the sum of the length of a copy sheet and the space interval at which copy sheets are fed, or a multiple of such sum. Therefore, in order to shorten said space interval of paper feeding for the purpose of increasing the copying speed, the diameter of the cylindrical screen should desirably be as small as possible. For these reasons, it is difficult to set up the diameters of the screen and the recording medium to equal values in an apparatus for which high speed operation is desired.